1. Field of the Invention
The invention, in general, relates to films or foils and products made from, or incorporating, foils or coatings and, more particularly, to such products made from starch or amylose derivatives of the kind useful in an office environment or as data carriers or image surfaces as well as teaching and learning materials such as, for instance, overhead projection slides, printing or drawing sheets, transparent folders or windows of envelopes. Other fields of application relate to the manufacture and refinement of special papers and to packaging materials as in see-through packages or parts of packages or as laminates on other web materials. Further fields of application of the foils here under consideration are in food packaging and agriculture.
2. The Prior Art
Films, foils and foil products, hereinafter sometimes collectively referred to as foils, are produced in huge quantities. Since a considerable amount of such foils are intended for one-time use only and since, following such use, they must be disposed, there is an increased demand for foils which can either be recycled or which are biodegradable. However, recycling is made difficult by the fact that these foils, because of their great variety, can often not be distinguished and sorted out by laymen for subsequent appropriate recycling processes. An alternative to such foils would be foils which are fully bio-degradable or compostible within a reasonably short time. Such foils would very likely eventually replace those foils which are not biodegradable or compostible, such as polyvinyl chloride, polystyrene, polyethylene, etc. provided they could be made to exhibit similar properties, especially in terms of transparency and mechanical strength and stability.
In the past, numerous experiments have been conducted in which starch served as the base or raw material for producing bio-degradable films or foils. Thus far, however, products made from pure starch have not been able effectively to compete with conventional products made from synthetic polymers. Their low tear strength and extensibility as well as lacking transparency and stability are drawbacks which have prevented use of those products on any significant scale.
To overcome the disadvantages and, more particularly, to improve the mechanical properties of starch-based foils, it has been proposed to add to the starch various softeners such as sorbite, glycerine, polyvinyl alcohol and so forth. Another way of improving the mechanical properties of starch-based products of the kind here under consideration is to add synthetic polymers based upon polyolefines. Thus, there is known a biodegradable plastic formed from corn starch and low-density polyethylene. The improvements brought about by such products have not, however, been such as to result in serious alternatives to synthetic polymers.
A further attempt to avoid, or at least lessen, the mentioned drawbacks was made by using so-called special amyloses such as, for instance, chemically modified high amyloses. For instance, certain hydroxyalkyl derivatives of amylose may be processed into thermoplastic articles or foils, films and membranes, or hydroxypropyl starches are added to other polymers, with a view to attaining improvements in predetermined properties. Thus, U.S. Pat. No. 3,850,652 relates to a specific application and teaches a multi-purpose membranous material of high flexibility and mechanical strength formed at least in part of a hydroxypropyl starch. Whilst this multi-purpose membranous material may be non-absorbable by other materials in contact therewith, it is neither transparent nor completely biodegradable.
Hydroxypropyl starches are also utilized for improving the biological degradation behavior of products made from unsaturated hydrocarbon polymer compounds which are difficult to decompose biologically. However, the properties of products thus obtained are by no means satisfactory. Whilst these products may be transparent, they nevertheless display some slight brownish tinge and they are of insufficient mechanical strength. Furthermore, U.S. Pat. No. 3,778,392 describes a mixture of polymeric starch and hyydropropyl starch for use in connection with high-quality papers. These products are not biodegradable and involve comparatively complex manufacturing processes.
As regards their utility, none of the known products derived from starch or amylose have been capable of competing effectively against comparable products made from synthetic polymers. Moreover, in most cases the additives and softeners added to the starch or amylose prevents complete biodegradability of the final product.